The importance of fertilizers in the growth of plants has long been recognized. Chemical fertilizers may be generally divided into two basic classes: primary fertilizers, and secondary fertilizers which are commonly referred to as trace elements or micronutrients. The primary fertilizers are nitrogen, comprising ammoniacal and nitrate nitrogen, phosphorus and potassium. Most fertilizer compositions sold commercially today include primarily these three elements. Such fertilizer compositions are often referred to as NPK fertilizers. In many agricultural applications for field grown crops the primary fertilizers are the only chemical fertilizers in the fertilizer composition, as it is expected that the small quantities of trace minerals or micronutrients necessary will be available from the soil. In other agricultural applications calcium will be added to the three primary fertilizers.
In many parts of the United States, and the world, container culture of nursery and greenhouse plants is expanding rapidly. These container and greenhouse plants present different requirements for proper fertilization. In order to achieve proper moisture drainage, and therefore oxygen for root development and nutrient absorption, a wide assortment of organic and inorganic materials are used as components of the container growing medium. This container growing medium takes the place of soil in which garden-grown or field-grown plants are produced. The importance of trace elements to obtain proper fertilization of container-grown plants is known and the growth of plants is substantially enhanced by the inclusion of trace elements in the fertilizer (in addition to the primary fertilizers). These trace elements may include iron, manganese, copper, zinc, boron, and molybdenum.
The ratio of nitrate to ammoniacal nitrogen supplied to a greenhouse plant should be varied at different times during the growing cycle. However, earlier methods of varying this ratio have not been systematic or continuous, but somewhat random using the fertilizer products commercially available. Furthermore, in varying the ratio of nitrate to ammoniacal nitrogen in a random manner, the micronutrient levels supplied to the plant have also been varied haphazardly, such that the plant is subjected to non-optimal and often deleterious levels of these nutrients.